Method for removing impurities from coal in a reaction chamber

ABSTRACT

A method for removing at least one impurity from coal is described herein. The method includes providing coal comprising a plurality of impurities and contacting the coal with an acid solution in a reaction chamber. At least one of the impurities reacts with the acid solution to produce one or more first products soluble in the acid solution. The method further includes removing at least a portion of the acid solution including at least a portion of the first products from the reaction chamber and adding a nitrate composition to the reaction chamber to form a nitrate solution. At least one of the impurities, at least one of the first products, or combinations thereof react with the nitrate composition to produce one or more second products soluble in the nitrate solution. The method still further includes removing at least a portion of the nitrate solution including at least a portion of the second products from the reaction chamber.

TECHNICAL FIELD

This disclosure generally relates to methods for producing ultra cleancoal, and more particularly relates to producing ultra clean coal byremoving impurities from coal.

BACKGROUND OF THE INVENTION

Clean coal, such as ultra clean coal, may be provided by treatment ofcoal including impurities to remove the impurities. For instance, coalincluding impurities may be treated with hydrofluoric acid in a firstreactor to produce a first reaction slurry. The first reaction slurrymay be transported to a filter (e.g., drum filter) to produce filtered,wet coal. The filtered, wet coal may then be transported to a secondreactor for treatment with nitrates to produce a second reaction slurry.The second reaction slurry may then be transported to a filter forfiltering. The resulting ultra clean coal may then be water washed andtransported to a dryer for drying. Thus, this multi-step processrequires multiple reactors, slurry pumps, and filters. The associatedexpense and space requirements for these various processing units may bequite large. Accordingly, there is a need for an improved process forremoving impurities from coal which avoid one or more of theaforementioned disadvantages and deficiencies.

SUMMARY OF THE INVENTION

This disclosure provides a method for removing at least one impurityfrom coal. The method includes providing coal comprising a plurality ofimpurities and contacting the coal with an acid solution in a reactionchamber. At least one of the impurities reacts with the acid solution toproduce one or more first products soluble in the acid solution. Themethod further includes removing at least a portion of the acid solutionincluding at least a portion of the first products from the reactionchamber and adding a nitrate composition to the reaction chamber to forma nitrate solution. At least one of the impurities, at least one of thefirst products, or combinations thereof react with the nitratecomposition to produce one or more second products soluble in thenitrate solution. The method still further includes removing at least aportion of the nitrate solution including at least a portion of thesecond products from the reaction chamber.

Other aspects, features, and advantages of this invention will beapparent from the following detailed description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow diagram illustrating a method 10 for producing ultraclean coal in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

As summarized above, this disclosure encompasses a method removing atleast one impurity from coal. Thus, embodiments of the method of thepresent disclosure provide ultra clean coal. As used herein, the term“ultra clean coal” refers to coal having a reduced ash content (e.g.,below about 2%) and/or a substantially reduced sulfur content such thatthe coal may be fed directly into processes such as gas turbineprocesses and provide advantages such as improved thermal efficiency,for example. As used herein, “ash” refers to both the non-combustiblecomponents in the coal before combustion and the non-combustiblebyproducts resulting from combustion of the coal. An embodiment of themethod for providing ultra clean coal is described below and illustratedin the flow diagram of FIG. 1. It should be understood that any system(e.g., gas turbine systems such as coal fired gas turbine systems,pulverized coal power plants, and integrated gasification combined cyclesystems) may use the coal (e.g., ultra clean coal) provided byembodiments of the methods of the present disclosure.

FIG. 1 is a flow diagram illustrating an embodiment of the method 10 forremoving at least one impurity from coal of the present disclosure. Themethod 10 includes providing coal comprising a plurality of impuritiesat step 12. Embodiments of method 10 may provide coal in the form ofanthracite coal, bituminous coal, subbituminous coal, lignite coal, orcombinations thereof In certain embodiments, the batches of coal treatedby the method 10 are sufficient for a 24-hour feed to a turbine. Inparticular embodiments, the coal is provided in batches ranging fromabout 300 tons to about 10,00 tons in the reaction chamber. In otherparticular embodiments, the coal is provided in batches ranging fromabout 400 tons to about 600 tons in the reaction chamber. In still otherparticular embodiments, the coal is provided in batches ranging fromabout 400 tons to about 500 tons in the reaction chamber. It should beunderstood, however, that the amount of coal provided in batches may bescaled up or down depending on the use (e.g., different turbines, adifferent number of turbines, different uses such as converting coal todiesel, etc.)

In some embodiments, the impurities include, but are not limited to,oxides of aluminum, iron, potassium, calcium, sodium, and other metals,minerals, inorganic and organic sulfur compounds, alkalis, ash, orcombinations thereof.

In certain embodiments, the impurities may be present in the coal in anamount ranging from about 2 wt. % to about 50 wt. %. In otherembodiments, the impurities may be present in the coal in an amountranging from about 3 wt. % to about 8 wt. %. In still other embodiments,the impurities may be present in the coal in an amount ranging fromabout 5 wt. % to about 7 wt. %.

TABLE 1 Examples of ranges of chemical composition for fly ash producedfrom different coal types (expressed as percent by weight). ComponentBituminous Subbituminous Lignite SiO₂ 20-60 40-60 15-45 Al₂O₃  5-3520-30 10-25 Fe₂O₃ 10-40  4-10  4-15 CaO  1-12  5-30 15-40 MgO 0-5 1-6 3-10 SO₃ 0-4 0-2  0-10 Na₂O 0-4 0-2 0-6 K₂O 0-3 0-4 0-4 LOI  0-15 0-30-5 Source: http://www.tfhrc.gov

The method 10 also includes contacting the coal with an acid solution ina reaction chamber at the second step 14. At least one of the impuritiesreacts with the acid solution to produce one or more first productssoluble in the acid solution. In some embodiments, the acid solution mayinclude, but is not limited to, hydrofluoric acid solution, a nitricacid solution, a hydrochloric acid solution, a hydrofluorosilicic acidsolution, a combination thereof, or other solutions of strong acids thatdissolve oxides.

An example of an additional reaction of the nitrate solution 28 with atleast one impurity is given below in Formula I.

SiO₂+4HF→SiF₄+2H₂O   (I)

In certain embodiments, the acid solution has an acid concentrationranging from about 3 M to about 10 M. In other embodiments, the acidsolution has an acid concentration ranging from about 3 M to about 6 M.In still other embodiments, the acid solution has an acid concentrationranging from about 4 M to about 6 M.

In particular embodiments, the weight ratio of acid solution to coal instep 14 ranges from about 10:1 to about 10:5. In other particularembodiments, the weight ratio of acid solution to coal in step 14 rangesfrom about 10:2 to about 10:4. In still other particular embodiments,the weight ratio of acid solution to coal in step 14 ranges from about10:2.5 to about 10:3.5.

In certain embodiments, the acid solution comprises a hydrofluoric acidsolution having a hydrofluoric acid concentration ranging from about 3 Mto about 10 M. In other embodiments, the acid solution comprises ahydrofluoric acid solution having a hydrofluoric acid concentrationranging from about 3 M to about 6 M. In still other embodiments, theacid solution comprises a hydrofluoric acid solution having ahydrofluoric acid concentration ranging from about 4 M to about 6 M. Inparticular embodiments, the weight ratio of hydrofluoric acid solutionto coal in step 14 ranges from about 10:1 to about 10:5. In otherparticular embodiments, the weight ratio of hydrofluoric acid solutionto coal in step 14 ranges from about 10:2 to about 10:4. In still otherparticular embodiments, the weight ratio of hydrofluoric acid solutionto coal in step 14 ranges from about 10:2.5 to about 10:3.5.

In particular embodiments, the second step 14 comprises contacting thecoal with the acid solution for about 1 hour to about 10 hours. In otherparticular embodiments, the second step 14 comprises contacting the coalwith the acid solution for about 3 hours to about 5 hours. In stillother particular embodiments, the second step 14 comprises contactingthe coal with the acid solution for about 4 hours to about 5 hours.

In particular embodiments, the second step 14 comprises contacting thecoal with the acid solution at a temperature ranging from about 70° F.to about 200° F. In other particular embodiments, the second step 14comprises contacting the coal with the acid solution at a temperatureranging from about 110° F. to about 170° F. In still other particularembodiments, the second step 14 comprises contacting the coal with theacid solution at a temperature ranging from about 140° F. to about 160°F.

In particular embodiments, the second step 14 comprises contacting thecoal with the acid solution at a pressure ranging from about 14 psia toabout 10 psia. In other particular embodiments, the second step 14comprises contacting the coal with the acid solution at a pressureranging from about 14 psia to about 42 psia. In still other particularembodiments, the second step 14 comprises contacting the coal with theacid solution at a pressure ranging from about 14 psia to about 20 psia.

In certain embodiments, the one or more first products comprises one ormore fluorides, hydroxides, hydroxyfluorides, oxides, or combinationsthereof. In embodiments where the first products comprise one or morefluorides, the fluorides may be selected from silicon fluoride, aluminumfluoride, iron fluoride, calcium fluoride, sodium fluoride, orcombinations thereof.

The third step 16 of the method 10 comprises removing at least a portionof the acid solution including at least a portion of the first productsfrom the reaction chamber. In particular embodiments, substantially allof the acid solution including substantially all of the first productsmay be removed from the reaction chamber in the third step 16.

In certain embodiments, the third step 16 may be carried out by openinga filter located proximate a bottom surface of the reaction chamber,thus allowing at least a portion of the acid solution including at leasta portion of the first products to flow through the filter and out ofthe reaction chamber.

The fourth step 18 of the method 10 comprises adding a nitratecomposition to the reaction chamber to form a nitrate solution such thatat least one of the impurities, at least one of the first products, orcombinations thereof react with the nitrate composition to produce oneor more second products soluble in the nitrate solution. In certainembodiments, the nitrate composition comprises an aqueous nitratesolution, nitric acid, aluminum nitrate, ferric nitrate, fluoronitrate,other nitrates, hydroxide, hydroxyl fluoride, hydroxynitrate, ionsthereof, or combinations thereof.

In some embodiments of the method, the one or more second productscomprises nitrate ions, sulfate ions, iron ions, hydroxyfluorides,oxides, fluoro nitrates, or combinations thereof.

Examples of additional reactions of the nitrate solution 28 with atleast one impurity are given below in Formula II and III.

FeS₂+14Fe(NO₃)₃+8H₂O→2SO₄ ²⁻+16H⁺+15Fe²⁺+42NO³⁻  (II)

SiF₄+2(Al,Fe)(NO₃)₃+2H₂O→SiO_(2(s))+2(Al,Fe)F₂ ⁺+4H⁺+6NO₃ ⁻  (III)

In particular embodiments, the nitrate composition has a concentrationranging from about 0.1 M to about 5 M. In other particular embodiments,the nitrate composition has a concentration ranging from about 0.1 M toabout 0.4 M. In still other particular embodiments, the nitratecomposition has a concentration ranging from about 0.1 M to about 0.3 M.

In certain embodiments, the weight ratio of nitrate composition to coalin step 18 ranges from about 10:1 to about 10:5. In other embodiments,the weight ratio of nitrate composition to coal in step 18 ranges fromabout 10:2 to about 10:4. In still other embodiments, the weight ratioof nitrate composition to coal in step 18 ranges from about 10:2.5 toabout 10:3.5.

In particular embodiments, the nitrate composition comprises a nitricacid solution having a nitric acid concentration ranging from about 0.1M to about 5 M. In other particular embodiments, the nitrate compositioncomprises a nitric acid solution having a nitric acid concentrationranging from about 0.1 M to about 0.4 M. In still other particularembodiments, the nitrate composition comprises a nitric acid solutionhaving a nitric acid concentration ranging from about 0.2 M to about 0.3M. In certain embodiments, the weight ratio of nitric acid solution tocoal in step 18 ranges from about 10:1 to about 10:5. In otherembodiments, the weight ratio of nitric acid solution to coal in step 18ranges from about 10:2 to about 10:4. In still other embodiments, theweight ratio of nitric acid solution to coal in step 18 ranges fromabout 10:2.5 to about 10:3.5.

According to certain embodiments of the present disclosure, the method10 further comprises maintaining the nitrate solution in the reactionchamber for about 20 hours to about 30 hours. In other particularembodiments, method 10 further comprises, maintaining the nitratesolution in the reaction chamber for about 22 hours to about 26 hours.

In particular embodiments, the method 10 further comprises maintainingthe nitrate solution in the reaction chamber at a temperature rangingfrom about 70° F. to about 190° F. In other particular embodiments, themethod 10 further comprises maintaining the nitrate solution in thereaction chamber at a temperature ranging from about 150° F. to about190° F. In still other particular embodiments, the method 10 furthercomprises maintaining the nitrate solution in the reaction chamber at atemperature ranging from about 140° F. to about 160° F.

In particular embodiments, the method 10 further comprises maintainingthe nitrate solution in the reaction chamber at a pressure ranging fromabout 14.4 psia to about 100 psia. In other particular embodiments, themethod 10 further comprises maintaining the nitrate solution in thereaction chamber at a pressure ranging from about 14.4 psia to about 43psia. In still other particular embodiments, the method 10 furthercomprises maintaining the nitrate solution in the reaction chamber at apressure ranging from about 14.4 psia to about 28 psia.

The fifth step 20 of the method 10 comprises removing at least a portionof the nitrate solution including at least a portion of the secondproducts from the reaction chamber. In particular embodiments,substantially all of the nitrate solution including substantially all ofthe second products may be removed from the reaction chamber in thefifth step 20.

In certain embodiments, the fifth step 20 may be carried out by openinga filter located proximate a bottom surface of the reaction chamber,thus allowing at least a portion of the nitrate solution including atleast a portion of the second products to flow through the filter andout of the reaction chamber.

After the fifth step 20, ash may be present in particular embodiments ofthe coal in an amount less than about 0.1% by weight. In certainembodiments of the method 10, after the fifth step 20, ash is present inthe coal in an amount of about 0.1% by weight to about 1% by weight. Inother embodiments of the method 10, after the fifth step 20, ash ispresent in the coal in an amount of about 0.1% by weight to about 0.3 %by weight.

In particular embodiments, the method further comprises agitating theacid solution in the reaction chamber, agitating the nitrate solution inthe reaction chamber, or both. In some embodiments, the method 10 alsocomprises, after the fifth step 20, water washing the coal.

By processing coal in one reaction chamber in a “batch” process toremove at least one impurity from coal, multiple reactors, multiplefilters, and conveying equipment (e.g., pumps and conveyer belts) may beeliminated and costs and space requirements may be reduced. In addition,exposure of coal outside of the reactor is reduced. In particular, onereaction chamber may be used to react both the first leaching solutionwith the coal and the second leaching solution with the coal. Thus,reducing coal losses and avoiding hazards associated with transfer ofchemicals is realized.

It should be apparent that the foregoing relates only to the preferredembodiments of the present application and that numerous changes andmodifications may be made herein by one of ordinary skill in the artwithout departing from the generally spirit and scope of the inventionas defined by the following claims and the equivalents thereof.

1. A method for removing at least one impurity from coal, the methodcomprising: providing coal comprising a plurality of impurities;contacting the coal with an acid solution in a reaction chamber, whereinat least one of the impurities reacts with the acid solution to produceone or more first products soluble in the acid solution; removing atleast a portion of the acid solution including at least a portion of theone or more first products from the reaction chamber; adding a nitratecomposition to the reaction chamber to form a nitrate solution, whereinat least one of the impurities, at least one of the first products, orcombinations thereof react with the nitrate composition to produce oneor more second products soluble in the nitrate solution; and removing atleast a portion of the nitrate solution including at least a portion ofthe one or more second products from the reaction chamber.
 2. The methodof claim 1, wherein the acid solution comprises a hydrofluoric acidsolution, a nitric acid solution, a hydrochloric acid solution, ahydrofluorosilicic acid solution, or combinations thereof.
 3. The methodof claim 1, wherein the acid solution comprises a hydrofluoric acidsolution having a hydrofluoric acid concentration of about 3 M to about10 M.
 4. The method of claim 1, wherein the acid solution comprises ahydrofluoric acid solution, and wherein a weight ratio of hydrofluoricacid solution to coal in the step of contacting is about 10:1 to about10:5.
 5. The method of claim 1, wherein the one or more first productscomprises one or more fluorides, hydroxides, hydroxyfluorides, oxides,or combinations thereof.
 6. The method of claim 5, wherein the one ormore first products comprises one or more fluorides selected from thegroup consisting of silicon fluoride, aluminum fluoride, iron fluoride,calcium fluoride, sodium fluoride, and combinations thereof.
 7. Themethod of claim 1, wherein the step of contacting comprises contactingthe coal with the acid solution for about 1 hour to about 10 hours. 8.The method of claim 1, wherein the step of contacting comprisescontacting the coal with the acid solution at a temperature ranging fromabout 70° F. to about 200° F.
 9. The method of claim 1, wherein thenitrate composition comprises nitric acid, ferric nitrate,fluoronitrate, hydroxide, hydroxyl fluoride, hydroxynitrate, ionsthereof, or combinations thereof.
 10. The method of claim 1, wherein thenitrate composition comprises a nitric acid solution having a nitricacid concentration of about 0.1 M to about 5 M.
 11. The method of claim1, wherein the nitrate composition comprises a nitric acid solution, andwherein the weight ratio of nitric acid solution to coal in the step ofadding is about 10:1 to about 10:5.
 12. The method of claim 1, whereinthe one or more second products comprises nitrate ions, sulfate ions,iron ions, hydroxyfluorides, oxides, fluoro nitrates, or combinationsthereof.
 13. The method of claim 1, further comprising, before the stepof removing the nitrate solution, maintaining the nitrate solution inthe reaction chamber for about 20 hours to about 30 hours.
 14. Themethod of claim 1, further comprising, before the step of removing thenitrate solution, maintaining the nitrate solution in the reactionchamber at a temperature ranging from about 70° F. to about 190° F. 15.The method of claim of 1, wherein the plurality of impurities compriseoxides of aluminum, iron, potassium, calcium, sodium, minerals,inorganic and organic sulfur compounds, alkalis, ash, or combinationsthereof.
 16. The method of claim 1, wherein, after the step of removingthe nitrate solution, ash is present in the coal in an amount less thanabout 0.1% by weight.
 17. The method of claim 1, wherein, after the stepof removing the nitrate solution, ash is present in the coal in anamount of about 0.1% by weight to about 1% by weight.
 18. The method ofclaim 1, further comprising agitating the acid solution in the reactionchamber, agitating the nitrate solution in the reaction chamber, orboth.
 19. The method of claim 1, further comprising, after the step ofremoving the nitrate solution, water washing the coal.
 20. The method ofclaim 1, wherein the coal is provided in batches of about 300 tons toabout 10,000 tons in the reaction chamber.